Cerebrospinal fluid (CSF) is a clear, plasma-like fluid that bathes the central nervous system (CNS). It occupies the central spinal canal, the ventricular system, and the subarachnoid space. It is produced mostly by the choroidal plexus of the ventricular system as well as its ependymal lining. The CSF has a physiologic volume of about 150 ml with a daily turn over of about 500 ml. Produced mostly in the lateral ventricles, the CSF passes through the interventricular foramen (of Monro), into the third ventricle. It exits the third ventricle through the cerebral aqueduct (of Sylvius) into the fourth ventricle. It exits the fourth ventricle into the cerebral subarachnoid space through the median aperture of Magendie and the two lateral apertures of Luschka. The CSF continues into the spinal subarachnoid space through the central canal of the spinal cord.
Normal CSF pressure (8 mmHg to 15 mmHg supine and about 20 mmHg erect) is a function of a tightly maintained equilibrium in rates of production and resorption of CSF. The out-pouching of the arachnoid mater called arachnoid granulations are responsible for the resorption of CSF into the dural venous sinuses. Disequilibrium in synthesis and resorption or obstruction of circulation results in CSF accumulation and raised intracranial pressure called hydrocephalus.